Probing thermochemical states during lean, laminar, flame-wall interaction using a triplet of simultaneous laser diagnostics

Abstract

Flame-wall interaction (FWI) is a phenomenon key to continued improvements in combustor efficiency and mechanical lifetimes. Needs have been identified in FWI research for multi-parameter measurements utilising simultaneous non-intrusive laser diagnostic systems. One such diagnostic, which is used simultaneously with others in an open-flame burner designed for probing the fundamental physics of FWI for the first time, is 1D coherent anti-Stokes Raman spectroscopy (CARS). 1D CARS has great potential for temperature measurements in combustion environments due to its high spatial resolution and single-shot measurement capabilities.

In this thesis, three laser diagnostic systems perform simultaneous single-shot measurements in the FWI region of a lean (Φ = 0.83) methane/air flame in a side-wall quenching burner under laminar (Re = 5900) conditions. These measurements aim to resolve open questions in FWI and surface quenching research, such as observing the hypothetical decoupling of the flame front position from the temperature gradient during wall heat loss; and understanding the relationship between reaction progress, gas temperature, and wall heat flux statistically in 2D. 1D hybrid fs/ps pure-rotational CARS measures 1D temperature profiles normal to the wall at various vertical positions. Planar laser-induced fluorescence of the OH radical (OH-PLIF) is used to locate the flame front in 2D based on the maximum gradient in OH signal. The quenching point is identified by the closest approach of this OH gradient to the wall. Finally, phosphor thermometry (PT) is used to measure 1D surface temperature profiles along the vertical centreline of the wall.

Beam-steering induced spatial averaging is investigated in the 1D CARS temperature profiles. An automated system for identification of spatially averaged CARS spectra is designed and successfully implemented. A procedure of partial spectral fitting to the high-wavenumber region above 158 cm-1 is used to correct the temperature biases induced by spatial averaging. This high-wavenumber fitting (HWF) process is checked for biases and validated. HWF successfully corrects even severe spatial averaging where temperature biases in regular spectral fits are > 1500 K.

Decoupling of the flame front and 1000 K isotherm is confirmed during FWI due to changes in the strong temperature gradient in the pre-heat zone ahead of the flame. The temperature gradient decreases during FWI, but briefly increases again when the pre-heat zone is compressed against the wall by the flame front during quenching. This event coincides with maximum wall heat flux, equal to (2.0 ± 0.3) × 105 W·m-2, and a rapid decline in flame front temperature from ~ 1700 K to < 1450 K. Flame quenching distances were found to agree with literature, with an ensemble-average value of 460 ± 38 μm.

The measurements presented in this thesis confirm that the adiabatic non-wall bounded relationships between the flame front and its thermal properties break down during FWI.